SUMMARY The majority of early-onset familial Alzheimer?s disease (AD) mutations are found in presenilin 1, PSEN1, the active component of ?-secretase. Most mutations increase levels of longer forms of ?-amyloid (A?) leading to enhanced deposition of ?-amyloid plaque and AD pathology. We will investigate a novel series of ?- secretase modulators (GSM) for their ability to pharmacologically reverse the pathological effects of these mutations. GSMs specifically modulate the cleavage activity of ?-secretase on APP processing to preferentially lower A?42 while increasing A?38 and A?37 levels, with minimal effects on A?40. Meanwhile, GSMs do not inhibit cleavage of other ?-secretase substrates, e.g. Notch. Particularly, we will study the effects of soluble GSM (SGSM), developed in our laboratory in collaboration with Dr. Steven Wagner (UCSD), on AD-related pathological events. We first reported the aryl 2-aminothiazole class of parent GSMs (AGSMs) (Kounnas et al., 2010). However, the AGSMs displayed poor aqueous solubility making them undesirable for clinical development. Subsequently, we developed water-soluble aryl 2-aminothiazole class, or 1st generation SGSMs (Wagner et al., 2014), with improved clinical potential. Recently, we structurally enhanced the aryl 2- aminothiazole SGSMs and developed the novel pyridazine class, of 2nd generation SGSMs, which display higher potency in inhibiting A?42 as compared to early SGSMs. Our original and current lead compounds in the pyridazine class are respectively SGSM-15606 and SGSM-776890, both displaying IC50 values of A?42 < 10 nM in cells. While SGSM-776890 is being prepared for a Phase-1A trial (single dose ascending), we have developed over a hundred analogs that have yet to be further tested as backups for clinical trials. In collaboration with other PPG projects, we will focus on the mechanism of actions of SGSM-15606, SGSM- 776890 and other analogs on AD pathology. Furthermore, we will focus on the roles of PSEN1 on adult hippocampal neurogenesis, a process in which human brain stem cells generate new neurons and glial cells throughout adulthood, which is impaired in AD, thereby contributing to cognitive deterioration in AD. Additionally, our GSMs have been provided to Project 1, which will focus on mechanisms by which PSEN1 mutations generate A? peptides ranging from 37-49 amino acids, Project 2, which will focus on the effects PSEN1 mutations on PSEN conformation and A?-dependent and A?-independent mechanism. Collectively, the results of the proposed studies not only will enhance our understanding of the biology of ?-secretase and the pathogenesis of AD, but also may identify a potential compound that can be a therapeutic for AD.